How many tvs in one household can log in and use on the individual plan?

There is no device limit, except for simultaneous streams.

• The Apple TV+ streaming service allows for up to 6 simultaneous streams per subscription. Enough for Family Sharing, one per group member when maximized. Download and stream shows and movies from Apple TV+ and Apple TV channels - Apple Support
• Apple Music allows for one stream at a time. Use the Family plan if you need more.

Apple TV+ is not limited to a specific number of 'screens', which is the model used for some other streaming services. You can view the content in any of your devices with the Apple TV app.

Apple provide these pages for support

Apple TV+

All the ways to watch Apple TV+

You can log multiple devices into your account. But, you can only watch one at a time.